1. Field of the Invention
The present invention relates to a wavelength calibration module and technique, and particularly to an absolute wavelength calibration module which optically absorbs at known wavelengths and detects such optical absorption when a narrowed emission band of an excimer or molecular laser is incident on the module.
2. Discussion of the Related Art
Excimer lasers emitting pulsed UV-radiation are becoming increasingly important instruments in specialized material processing. The KrF-excimer laser emitting around 248 nm and the ArF-excimer laser emitting around 193 nm are rapidly becoming the light sources of choice for photolithographic processing of integrated circuit devices (IC's). The F.sub.2 -laser is also being developed for such usage and emits light around 157 nm.
It is important for their respective applications to the field of submicron silicon processing that each of the above excimer laser systems become capable of emitting a narrow spectral band around a very precisely determined and finely adjustable absolute wavelength. Techniques for reducing bandwidths by special resonator designs to less than 100 pm, and in some cases to less than 1 pm, are well known. Techniques are also available for tuning and controlling central wavelengths of emission. However, most of these techniques do not accurately determine absolute wavelengths and only serve to relatively tune and control wavelengths. Moreover, even relative wavelength changes cannot be as precisely determined as is desired, using these techniques.
It is possible to roughly determine an absolute wavelength or a change in wavelength from a reference wavelength as a spectral band is tuned, when particular incremental settings of a spectrograph are calibrated to correspond to absolute wavelengths in conventional units, e.g., in nanometers. However, conventional techniques do not provide very precise absolute wavelength and incremental wavelength change information at any time. This is because a conventional spectrograph often must undergo a laborious conventional calibration technique. Moreover, optical drift and other optical, thermal and electronic phenomena produce uncertainty and imprecision at all times following the calibration procedure, including during operation of the system. Further, wavelength calibration is usually done externally to the operating beam path of the system using high resolution spectrographs in combination with spectral reference tools for wavelength calibration, e.g., spectral lamps emitting particular narrow lines. Therefore, very precise and temporally advantageous absolute wavelength determination and fine tuning methods are needed.
Specifically, it is desired to have absolute wavelength calibration techniques for UV-emitting excimer and molecular lasers having accuracies within a range of .+-.0.25 pm, while having tuning versatility comprising wavelength ranges from .+-.5 pm to greater than .+-.100 pm depending on properties of available illumination tools for IC production. There are available techniques for accurately determining the absolute wavelength of a narrow band KrF-excimer laser emission using narrow spectral absorption lines of certain elements to calibrate a high resolution spectrometer. Among these available techniques, atomic transition(s) of iron (Fe) at 248.327 and/or 248.4185 mn are used to detect absorption signals either by reduced optical transmission or using the opto-galvanic effect. See U.S. Pat. No. 4,823,354 to Znotins et al.; U.S. Pat. No. 5,450,207 to Fomenkov; F. Babin et al., Opt. Lett., v. 12, p. 486 (1987); See also R. B. Green et al., Appl. Phys. Lett., v. 29, p. 727 (1976) (describing galvanic detection of optical absorptions in a gas discharge for various gases including lithium (Li), sodium (Na), uranium (U) and barium (Ba)).
Babin et al. discloses using the opto-galvanic effect to determine the KrF-laser absolute emission wavelength. A galvatron having an anode and a cathode is set in the optical path of the laser beam. An Fe vapor fills the galvatron. A voltage is monitored between the cathode and anode. The emission wavelength of the laser is narrowed and tuned through a range around 248 nm. When the wavelength of the beam impinging the Fe-vapor filled gas volume between the cathode and the anode corresponds to an atomic transition of Fe, a resonance between the levels causes a marked change in voltage between the anode and cathode. Since the absorption lines of Fe are well known and consistent, the absolute wavelength of the narrowed laser emission band is determinable.
Znotins et al. and Fomenkov each disclose using a photodetector to detect the intensity of light emitted from a KrF-laser. Znotins et al. discloses to use a galvatron having benzene vapor inside. Fomenkov discloses to use a galvatron having an Fe cathode inside. The cathode of Fomenkov gives off Fe vapor which fills the galvatron when a current is generated between the cathode and an associated anode. Light emitted from the KrF-laser traverses the gaseous benzene or iron medium of the galvatron before impinging the photodetector. When the wavelength corresponds to an atomic transition of the gas medium of the galvatron, the gas absorbs the light, and the intensity of light detected is reduced. Thus, the absolute wavelength of emission of the KrF-laser is determinable in also determinable in this alternative way.
The opto-galvanic effect described by Babin et al. and acknowledged by Fomenkov permits a very precise and reliable determination of an absolute emission wavelength of a KrF-excimer laser system. See U.S. Pat. No. 4,905,243 to Lokai et al. A known technique uses sealed hollow cathode lamps containing Fe-vapor in a Ne-buffer gas environment. See Hammamatsu Datasheet: Opto-Galvanic Sensor, Galvatron L 2783 Series, November 89, Japan. Thus, the Fe-lamp has become an important and reliable measuring tool for absolute wavelength calibration for KrF-lithography laser systems in the 248 nm spectral region.